Effects of rank and calcium catalysis on oxygen chemisorption and gasification reactivity of coal chars

The effects of coal rank and calcium catalysis on oxygen gasification of coal chars have been investigated. Five different coals, from lignite to anthracite were used. Coals were demineralized and a calcium catalyst was deposited on the carbon in different amounts, by ion exchange for lignite and subbituminous coals and by impregnation for the others. Chars from all coals were obtained by both slow and rapid pyrolysis.; Oxygen chemisorption studies conducted under conditions far away from gasification and measured oxygen uptakes during gasification revealed that large amounts of oxygen are chemisorbed. The lower the coal rank, the greater the amount of chemisorbed oxygen in both cases. The presence of a calcium catalyst additionally increased the oxygen uptake by solid carbons. The chemisorption tests also showed the influence of diffusion inside the smallest micropores on the kinetics of the process.; Reactivity profiles were investigated in detail. Demineralized coal chars showed monotonic, linear increases with burn-off for a broad range of conversion (20-80%). The higher the coal rank, the greater the reactivity increase per unit burn-off. A comparison of reactivities of the demineralized form of coal chars confirmed that the reactivity is affected by diffusion inside the smallest micropores for experiments in the intermediate temperature range, usually 700-800 K.; A comparison of reactivities of the calcium-loaded and demineralized coal chars prepared and subsequently reacted at the same conditions has confirmed that the catalytic effect of calcium is the greatest for lower-rank coals, and that it decreases with increasing coal rank. Comparable reactivities for as-received and calcium-loaded lignite and subbituminous char were about two orders of magnitude greater than for a corresponding demineralized char. For higher ranks of coal the effect of calcium loading is smaller than one order of magnitude. For the lower ranks of coal, where calcium is very well dispersed, reactivity profiles are confirmed to be dominated by the catalytic effect.; Based on the reactivity and oxygen chemisorption studies, it was concluded that the effect of oxygen diffusion on char reactivity is much greater for higher-rank coals than for lower-rank coals. For the lignite char the diffusion effect is only important at the beginning of gasification and it decreases with increasing burn-off. For the anthracite char it is about 3 times greater at the very low burn-offs than at 85% burn-off. In addition, for demineralized anthracite char this diffusion effect lasts longer in terms of time and conversion.